1. Field of the Invention
The present invention relates to an injection-molding mold die for injecting a molten material at a predetermined pressure into a cavity formed by clamping the mold die.
2. Description of the Related Art
Many items are molded by injection-molding with which a material melt into a cavity formed by clamping a mold die is injected at a predetermined pressure. With respect to this injection-molding, mold designed are performed so as to cope with items having a variety of shapes; hence, an item having even a shape like an undercut which cannot be drafted in the direction of opening the mold die is designed so as to be molded.
A dynamic-pressure bearing molded by injection-molding will be described by way of example.
FIG. 1 illustrates an example dynamic-pressure bearing 8, a part of which is cut out.
The dynamic-pressure bearing 8 shown in FIG. 1 has a cylindrical shape and includes dynamic-pressure grooves 81 having a herringbone shape (a shape in which grooves reversed in a dog-legged shape are arranged at a predetermined interval in the circumferential direction), formed in two above and below rows on its internal peripheral surface 80 coming into contact with a rotating shaft 9 indicated by a dotted-chain line in the figure. Since the dynamic-pressure grooves 81 hold lubricant, when the rotating shaft 9 rotates, the lubricant is drawn to the reversed portion of the dynamic-pressure grooves 81, thereby causing the reversed portion to have a high oil-film pressure generated thereat. The dynamic-pressure bearing 8 generates a load toward the center with this hydraulic pressure, thereby bringing about the rotating shaft 9 and the internal circumferential surface 80 in a non-contact state and rotatably supporting the rotating shaft 9.
In order to form the herringbone dynamic-pressure grooves 81 on the internal peripheral surface 80 during injection-molding, a core pin having undulations for forming dynamic-pressure grooves on the external peripheral surface thereof is used. Meanwhile, since a herringbone shape is formed of undercuts, when a dynamic-pressure bearing itself possibly has a cut formed therein so that the internal diameter thereof is likely to become larger due to its elastic deformation (for example, see Japanese Unexamined Utility Model Registration Application Publication No. 5-76721), the undercuts can be drafted from the core pin; unfortunately, with this idea, the cut breaks up the circumferential continuity of the dog-legged reversed grooves forming a herringbone shape, thereby resulting in failure of having a high oil film pressure generated in the grooves. In view of this, by devising the design of the mold die, a so-called divided-molding method is proposed in which a cylindrical dynamic-pressure bearing is made by independently injection-molding two semi-cylinders, each having a shape that a cylinder is divided into two parts along its center axis, and by bonding them to each other (for example, see Japanese Unexamined Utility Model Registration Application Publication No. 5-47534). Also, a so-called rotationally drafting method is proposed in which a core pin divided into two parts so that the reversed portion of the herringbone-shaped grooves serves as a parting line is used so as to form the dynamic-pressure groove portion of the dynamic-pressure bearing while rotating the core pin (for example, see Japanese Unexamined Patent Application Publication Nos. 2000-240640 and 7-110028). In addition, a so-called forcefully extracting method is proposed in which the dynamic-pressure groove portion of the dynamic-pressure bearing is drafted from the core pin by utilizing elastic deformation of the dynamic-pressure bearing as a molded item (for example, see Japanese Unexamined Patent Application Publication Nos. 7-190048, 10-331841, 11-311242, and 2001-65570, and Japanese Patent No. 3056024). Furthermore, a so-called diameter-variance method is proposed in which the dynamic-pressure groove portion of the dynamic-pressure bearing is drafted by utilizing a difference in coefficients of linear expansion of a core pin and a dynamic-pressure bearing as a molded item (for example, see Japanese Unexamined Patent Application Publication Nos. 63-203916 and 5-312213).
A dynamic-pressure bearing is incorporated in a small precision apparatus such as a hard-disk drive since non-repeatable run-out (NRRO) asynchronous with rotation of its rotating shaft is reduced and its silence property is also improved. The depth of each dynamic-pressure groove of the dynamic-pressure bearing incorporated in such a small precision machine is on the order of several micrometers to tens of micrometers.
Hence, with the foregoing divided-molding method disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 5-47534, precise alignment of the bonding surfaces is difficult. Also, with the rotational drafting method disclosed in Japanese Unexamined Patent Application Publication Nos. 2000-240640 and 7-110028, since the reversed portion of the herringbone-shaped grooves serves as a parting line, when dynamic-pressure grooves having a depth on the order of several micrometers to tens of micrometers are intended to be precisely formed, die-matching is difficult. In addition, because of a shape restriction, a dynamic-pressure bearing cannot be made so as to form a shape that herringbones are provided in two rows as shown in FIG. 1, for example. Further, with the forcefully drafting method disclosed in Japanese Unexamined Patent Application Publication Nos. 7-190048, 10-331841, 11-311242, and 2001-65570, and Japanese Patent No. 3056024, since the grooves have a depth on the order of several micrometers to tens of micrometers, there is a risk that the grooves are deformed or crushed when the dynamic-pressure groove portion of the dynamic-pressure bearing is drafted from a core pin.
Also, when dynamic-pressure grooves having a depth on the order of several micrometers to tens of micrometers are intended to be precisely formed, materials available for the dynamic-pressure bearing are limited due to their properties; hence, it is needed to use polyphenylene sulfide (PPS), liquid crystal polymer (LCP), or the like, which is called super-engineering plastic. Referring to Table 1, an example dynamic-pressure bearing formed so that its internal diameter is designed at 3.5 mm and its internal diameter is expanded more than the external diameter of a core pin by heating the dynamic-pressure bearing up to 80° C. will be described.
TABLE 1FactorsLPC withPPS withPOM with non-ResinUnitreinforcedreinforcedreinforcedmaterial—sliding gradesliding gradegradeChange inmm0.0040.0060.030diametersCoefficient1/K1.2 × 10−52.0 × 10−51.1 × 10−4of linearexpansion
In the leftmost, and the center, columns of the factor column, Table 1 shows differences in the external diameter of the core pin and the internal diameters of dynamic-pressure bearings (changes in diameters) injection-molded with LPC, PPS included in super-engineering plastic as a molten material, respectively. In the rightmost column shows the same with polyacetal (POM) included in general-purpose engineering plastic. Also, Table 1 shows coefficients of linear expansions of the respective molten materials.
As shown in Table 1, when LCP or PPS is used as a molten material, the change in diameters is smaller than that when POM is used, whereby it is difficult to draft the dynamic-pressure groove portion of the dynamic-pressure bearing while expecting the change in diameters. Meanwhile, when POM having a greater coefficient of linear expansion than that of LCP or PPS is used as a molted material, a change in diameters can be expected to a certain extent. However, a material having a large coefficient of linear expansion generally does not allow an item to be accurately molded; hence, when a material having a large coefficient of linear expansion allowing a certain amount of change in diameters is used as a molten material, it is difficult to accurately form dynamic-pressure grooves on the order of several micrometers to tens of micrometers.
As described above, even when any one of the molding methods is used, a dynamic-pressure bearing is not accurately molded. In view of the above problems, another method is proposed in which undulations for forming dynamic-pressure grooves are provided on the external peripheral surface of a sleeve composed of a heat-resistant rubber; the external diameter of the sleeve is enlarged by inserting a rigid body having an external diameter greater than the internal diameter of the sleeve into the sleeve; and a dynamic-pressure bearing is injection-molded in a state in which the external diameter of the sleeve is enlarged (see Japanese Unexamined Patent Application Publication No. 9-57760). According to the method of using the sleeve composed of the heat-resistant rubber, when the rigid body is drafted from the sleeve on the occasion of opening the mole die, the external diameter of the sleeve becomes smaller due to elasticity of the sleeve, whereby the dynamic-pressure groove portion of the dynamic-pressure bearing is drafted. An example of heat-resistant rubber practically available for the sleeve is a heat-resistant fluoro rubber (FKM). The heat-resistant temperature of the sleeve in continuous use is about 200° C., and its high-temperature limit in use is on the order of 260° C. to 300° C. The heat-resistant FKM has a longitudinal elastic modulus on the order of 0.5 MPa to 8 MPa, which is far smaller than that of a metal material normally applied for a mold die, and is thus likely to be deformed.
In order to accurately form dynamic-pressure grooves on the order of several micrometers to tens of micrometers, when a super-engineering plastic is used as a molten material for example, its injection-molding temperature is likely to be rather high of about 200° C. to 300° C. Also, its injection-molding pressure is about 10 MPa to 80 MPa. Accordingly, the sleeve composed of the heat-resistant rubber disclosed in Japanese Unexamined Patent Application Publication No. 9-57760 is insufficient in thermal and strength aspects for accurately injection-molding a precision component.